http://wiki.zimbra.com/wiki/Performance_Tuning_Guidelines_for_Single-Server_100-500_User_Systems

http://wiki.zimbra.com/wiki/Category:Performance_and_Tuning

Stop amavisd:

su - zimbra
zmamavisdctl stop
exit

Edit fstab:

vim /etc/fstab

Add:

# Mount amavis temp to RAM disk
tmpfs        /opt/zimbra/data/amavisd/tmp              tmpfs   defaults,noexec,nodev,nosuid,size=1024m,mode=750,uid=zimbra,gid=zimbra 0 0

Restart amavisd:

su - zimbra
zmamavisdctl start